title: MRI acquisition and analysis protocol for in vivo intraorbital optic nerve segmentation at 3T.
abstract: OBJECTIVE
  To present a new acquisition and analysis protocol for reliable and reproducible segmentation of the entire intraorbital optic nerve (ION) mean cross-sectional area by means of magnetic resonance imaging (MRI) at 3 tesla (T).
  
  
  METHODS
  Eight healthy volunteers (mean age 31, five were male) gave written informed consent and both of their IONs were imaged individually using a coronal-oblique T2-weighted fast multidynamic image acquisition scheme; the proposed acquisition scheme has its rationale in combining separately acquired volumes and registering them to account for motion-related artifacts commonly associated with longer acquisitions. Mean cross-sectional area of each ION was measured using a semiautomated image analysis protocol that was based on an active surface model previously described and used for spinal cord imaging. Reproducibility was assessed for repeated scans (scan-rescan) and repeated image analysis performance (intraobserver).
  
  
  RESULTS
  Mean and SD values of the left ION cross-sectional area for the eight healthy volunteers were 5.0 (±0.7) mm² and for the right ION were 5.3 (±0.8) mm². Mean scan-rescan coefficient of variation (COV) for the left ION was 4.3% and for the right was 4.4%. Mean intraobserver COV for the left ION was 2.1% and for the right was 1.8%.
  
  
  CONCLUSIONS
  This study presents a new MRI acquisition and analysis protocol for reliable and reproducible in vivo measurement of the entire ION mean cross-sectional area as demonstrated in a pilot study of healthy subjects. The protocol presented here can be used in future studies of the ION in disease state.
authors.affiliation: Nuclear Magnetic Resonance Research Unit, University College London Institute of Neurology, London, United Kingdom. m.yiannakas@ucl.ac.uk
authors.forename: Marios C
authors.initials: MC
authors.lastname: Yiannakas
authors.forename: Ahmed T
authors.initials: AT
authors.lastname: Toosy
authors.forename: Rhian E
authors.initials: RE
authors.lastname: Raftopoulos
authors.forename: Raj
authors.initials: R
authors.lastname: Kapoor
authors.forename: David H
authors.initials: DH
authors.lastname: Miller
authors.forename: Claudia A M
authors.initials: CA
authors.lastname: Wheeler-Kingshott
mesh_terms: D000328:Adult; D019412:Anatomy, Cross-Sectional; D016477:Artifacts; D005260:Female; D006801:Humans; D007091:Image Processing, Computer-Assisted / Q000379:methods*; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008297:Male; D009900:Optic Nerve / Q000033:anatomy & histology*; D009915:Orbit / Q000033:anatomy & histology*; D010865:Pilot Projects; D015203:Reproducibility of Results
keywords: MRI; optic nerve; segmentation